Semiconductor device and manufacturing method thereof

ABSTRACT

A semiconductor device includes a first wiring extending in a first direction and a second wiring extending in a second direction which crosses the first direction and being disposed with a space interposed between the first wiring and the second wiring, and including a tantalum layer, a tantalum nitride layer formed over the tantalum layer, and a metal layer formed over the tantalum nitride layer.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2008-221412, filed on Aug. 29,2008, the entire contents of which are incorporated herein by reference.

FIELD

The present invention relates to a semiconductor device and amanufacturing method thereof.

BACKGROUND

It is desirable that semiconductor devices, for example field effecttransistors for power amplification, be miniaturized so that suchsemiconductor devices may be manufactured at lower costs.

However, a distance between wirings, more specifically the distancebetween the wirings which intersect with each other with an insulationfilm interposed between the wirings, may be narrowed due to theminiaturization of the semiconductor device. Such narrowing between thewirings causes a larger parasitic capacitance between the wirings andmakes a high frequency operation difficult in the semiconductor device.

An air-bridge wiring has been effectively used to reduce the parasiticcapacitance between wirings intersecting with each other with theinsulation film interposed between the wirings. Especially, theair-bridge wirings are effective in achieving high speed operations ofthe field effect transistors for power amplification in which a numberof field effect transistors (FETs), called a “multi-gate transistor,”are provided over the same semiconductor substrate.

FETs are arranged in one line, and terminals (sources, drains, andgates) are coupled to other corresponding terminals by comb-shapedelectrodes to form the multi-gate transistor. In such a multi-gatetransistor, intersections are inevitably formed at locations wheresource electrodes are coupled with a plurality of sources and gateelectrodes are coupled with a plurality of gates.

To avoid contact of a gate electrode and a source electrode, a bridgewiring structure is provided at the intersection. The bridge wiringstructure is a structure where one electrode crosses over the otherelectrode with an overhead crossing with an insulation film beinginterposed between both electrodes (For example, see Japanese Laid-OpenPatent Application 2003-197740.).

The parasitic capacitance becomes greater in the bridge wiring structuredue to a high dielectric constant of the insulation film providedbetween the crossing wirings. In consequence, air-bridge wirings wherethe crossing wirings are separated by a space are used in achieving thehigh-speed operations of the multi-gate transistor.

Gold (Au) having a high electric conductivity is widely used as a metallayer that forms the air-bridge wiring. However, gold is a soft metaland maintaining the air-bridge structure is difficult for gold alone.Consequently, a laminated structure that includes titanium (Ti) andplatinum (Pt) is used as a support body to form a Ti/Pt/Au laminatedstructure, and the air-bridge wiring is formed. Here, Ti is used toachieve better adhesion between the substrate and the air-bridge layer,and the Au layer is supported by a Pt layer (For example, see JapaneseLaid-Open Patent Application 2007-150282.).

In recent years, high electron mobility transistors (HEMT) in whichchannel layers are formed of gallium nitride (GaN) have been attractingmuch attention as a high frequency and high output transistors(Hereinafter, a HEMT whose channel layer is formed of gallium nitride(GaN) is referred to as a “GaN-HEMT”.).

Since the band gap of GaN is wider in comparison with those of Si andGaAs, the GaN-HEMT is suitable for operations under high temperatures.Moreover, since the breakdown voltage of the GaN-HEMT is high, theGaN-HEMT is suitable for operations at high voltages. Consequently,GaN-HEMTs may have fewer malfunctions due to an increase in operationtemperature or an increase in electrical field, even when the GaN-HEMTsare miniaturized and operated under a large current.

For this reason, multi-gate transistors which include GaN-HEMTs are usedas high frequency/high output power amplifiers.

SUMMARY

Accordingly, it is an object of an aspect of the invention to provide asemiconductor device including a first wiring extending in a firstdirection and a second wiring extending in a second direction whichcrosses the first direction and being disposed with a space interposedbetween the first wiring and the second wiring, and including a tantalumlayer, a tantalum nitride layer formed over the tantalum layer, and ametal layer formed over the tantalum nitride layer.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an enlarged view of a left side end of a Ti/Pt/Auair-bridge wiring;

FIG. 2 illustrates an enlarged view of the area A circled by the dottedline in FIG. 1;

FIG. 3 is a graph representing relationships between concentrations ofnitrogen in TaN and reaction times;

FIG. 4 illustrates a cross sectional view of a Ti/TaN/Au air-bridgewiring;

FIGS. 5A to 5H illustrate cross sectional views of a manufacturingprocess of the Ti/TaN/Au air-bridge wiring;

FIG. 6 illustrates a structural perspective view of a multi-gatetransistor according to a first embodiment;

FIG. 7 illustrates a cross sectional view of the multi-gate transistoralong the line A-A in FIG. 6;

FIG. 8 illustrates a cross sectional view of the multi-gate transistoralong the line B-B in FIG. 6;

FIGS. 9A to 9I illustrate cross sectional views of a manufacturingprocess of an air-bridge wiring structure according to the firstembodiment;

FIG. 10 is a graph representing results of accelerated thermaldegradation tests implemented on a Ta/TaN laminated film according tothe first embodiment;

FIG. 11 illustrates a cross sectional view of an air-bridge wiring thatforms a multi-gate transistor according to a second embodiment;

FIG. 12 is a graph representing results of accelerated thermaldegradation tests implemented on a Ta/Ti/TaN laminated film according tothe second embodiment;

FIG. 13 illustrates a structural perspective view of a multi-gatetransistor according to a third embodiment;

FIG. 14 illustrates a cross sectional view of the multi-gate transistoralong the line B-B in FIG. 13;

FIGS. 15A to 15D illustrate cross sectional views indicating amanufacturing process of the multi-gate transistor according to thethird embodiment;

FIG. 16 illustrates a circuit diagram of a high frequency amplifieraccording to a fourth embodiment; and

FIG. 17 illustrates a block diagram of a transmitter according to afifth embodiment.

DESCRIPTION OF EMBODIMENTS

An air-bridge wiring capable of reducing a parasitic capacitance is usedin forming a multi-gate transistor with FETs suitable for high frequencyoperations. Here, a large current is caused to flow through theair-bridge wiring when the multi-gate transistor is formed with FETs,such as a GaN-HEMT, that operate with the large current. Inconsideration of the facts disclosed above, the inventor studied whatmight occur when an output of a multi-gate transistor that includes aplurality of GaN-HEMTs (hereinafter, referred to as a “GaN-HEMTmulti-gate transistor”) was made higher.

The study revealed that the large current flowing through the air-bridgewiring might cause electromigration of Au forming the air-bridge wiring,and Au might enter grain boundaries of a Pt support layer. In the abovecase, collapse of the Pt support layer may occur, and since theair-bridge wiring loses its support body, and this may result in wiringdisconnection.

Hereinafter, embodiments of the present invention will be disclosed withreference to the accompanying drawings. It should be noted that thescope of the present invention is not limited to the embodimentshereinafter disclosed.

The main body of the air-bridge wiring is a thick Au layer if theair-bridge wiring is formed of a Ti/Pt/Au laminated structure. Since Auis a soft metal, the Au layer alone may not fully maintain an air-bridgestructure. Consequently, a Pt layer formed under the Au layer supportsthe Au layer.

Meanwhile, it is well known that Au has a tendency to causeelectromigration. For the above reason, Au atoms ejected by an electronflow may migrate into a Pt film from the Au layer, and the migration maycause the collapse of the Pt film when the multi-gate transistor isoperated under a large current, for example, equal to or more than 1×10⁵A/cm².

FIG. 1 is a view that explains electromigration. FIG. 2 is a view wherean area “A” circled by the dotted line in FIG. 1 is enlarged.

FIG. 1 illustrates a left side end of an air-bridge wiring 12. Here, theair-bridge wiring 12 is formed over a common electrode 14 coupled to aplurality of gates of the multi-gate transistor with a space 38 beinginterposed between the air-bridge wiring 12 and the common electrode 14.In addition, the air-bridge wiring 12 is formed by laminating a Ti layer40, a Pt layer 42, a first Au layer 44, a second Au layer 46, and athird Au layer 48.

As illustrated in FIG. 1, an electron flow 60 passing through theair-bridge wiring 12 moves straight ahead in the vicinity of an innerwall 62 of the air-bridge wiring 12, and thereafter, the electron flow60 changes direction (Note that FIG. 1 illustrates the electron flowflowing along the bottom of the air-bridge wiring 12.). Consequently, asillustrated in FIG. 2, Au atoms 64 of the first to the third Au layers44, 46, and 48, which collide with the electron flow 60 and aredispersed, migrate toward the Pt layer 42 (in other words, conductelectromigration.).

On the other hand, a large number of grain boundaries 66 extending in agrowth direction exist in the Pt layer 42. The Au atoms 64 migratingtoward the Pt layer 42 enter the grain boundaries 66 and try to breakthe bonds between microcrystals of Pt.

As a result, the Pt layer 42 may collapse, and the Pt layer 42 may beunable to support the air-bridge wiring 12. Consequently, it may beassumed that the air-bridge wiring 12 that loses its support body maybecome disconnected.

Note that although the air-bridge wiring 12 illustrated in FIGS. 1 and 2bends at a right angle, it is assumed that similar phenomena may occurand result in the disconnection of the air-bridge wiring 12 even incases where the air-bridge wiring 12 bends at lesser angles.

Metal atoms exposed to the electron flow strongly oscillate due torepeated collisions with electrons and, as a result, the metal atomsmigrate from lattice points that bind the metal atoms. Although thedirections of the migration of individual metal atoms are not constantat this point of time, many of the metal atoms, as a group, migratetoward a downstream side of the electron flow. Such a migrationphenomenon of the metal atoms is referred to as “electromigration.”

Oscillation of the metal atoms that induces the electromigration issimilar to oscillations of metal atoms heated at high temperatures.Therefore, it is assumed that metals stable at high temperatures, inother words metals with high melting points, may have a highelectromigration resistance.

The inventor studied the electromigration resistance of Ta_(1−x)N_(x)(0≦x<1) that is a high-melting-point metal capable of being used as theair-bridge wiring 12.

The electromigration resistance was studied based on the acceleratedthermal degradation test in which metal films under test were heated andthe rates of degradation of the heated metals were measured. The testmay create a state that is similar to the state in which metal atoms areexposed to the electron flow and strongly oscillated, by exposing themetal films under test to high temperatures.

The following experiments were conducted. A specimen(Ti/Ta_(1−x)N_(x)/Au laminated film, 0≦x<1) was created by forming aTi/Al laminated film in which an Al layer was laminated over a Ti layer;and a Ti layer, a Ta_(1−x)N_(x) layer (0≦x<1), and an Au layer werelaminated in order over the Ti/AL laminated film. Furthermore, acomparison specimen (Ti/Pt/Au laminated film) was created by laminatinga Ti layer, a Pt layer, and an Au layer, in order, over the Ti/Allaminated film.

Here, the Ta_(1−x)N_(x) layer is formed by a reactive sputtering methodin which Ta is used as a target. A sputter gas and a reactive gas maybe, for example, Ar and N₂, respectively. The Au layer and the Pt layerare formed by the sputtering method in which Au and Pt are used astargets, respectively. A sputter gas used here may be, for example, Ar.

Here, the thicknesses of the Ti layer, the Ta_(1−x)N_(x) layer, and theAu layer may be, for example, several tens of nanometers (nm), 200 nm,and 50 nm to 300 nm, respectively. On the other hand, the thickness ofthe Pt layer of the comparison specimen is, for example, 200 nm that issimilar to, if not the same as, the Ta_(1−x)N_(x) layer. Note that thethicknesses of the Al layer and the Ti layer are similar, if not thesame, in all the specimens.

The specimens formed in the manner disclosed above were heated up to450° C. and the time until changes appear on the surface of the Au layerwas measured. Here, the thickness of the Au layer is configured so thatthe changes may easily appear on the surface of the Au layer.

The surfaces of the specimens were uniform and without characteristicsbefore the specimen is heated. When, however, the specimen wascontinuously heated, eventually spots appeared on the surface of thespecimen. Heating times (hereinafter, referred to as a “reaction time”)until the spots appear were measured and compared.

The spots were considered to bean AlAu alloy generated by the Au atomsthat broke the Ta_(1−x)N_(x) layer (or the Pt layer) and reacted withthe Al film provided under the Ta_(1−x)N_(x) layer (or the Pt layer).That is to say, the reaction time measured based on the acceleratedthermal degradation test represents the time taken for the stronglyoscillating Au atoms to damage the Ta_(1−x)N_(x) layer (or the Ptlayer). Hence, it is assumed that longer the reaction time of a metalfilm, the higher the electromigration resistance of the metal film.

FIG. 3 is a graph that represents the measurement results of thereaction times, where the composition of the Ta_(1−x)N_(x) layer isvaried. The horizontal axis of the graph represents a concentration ofnitrogen in the Ta_(1−x)N_(x) layer. The vertical axis thereofrepresents the reaction time. The curved line “TaN” represents thereaction time of the specimen that has the Ti/Ta_(1−x)N_(x) laminatedfilm formed between the Ti/Al laminated film and the Au layer. On theother hand, the dotted line “Pt” represents the reaction time(approximately 2 minutes) of the specimen where a Ti/Pt laminated filmis formed between the Ti/Al laminated film and the Au layer. Here, sincethe Ti/Ta_(1−x)N_(x) laminated film and the Ti/Pt laminated film bothinclude a Ti layer, the Ti layer does not cause a difference betweenreaction times of either of the specimens.

As represented by the graph of FIG. 3, the reaction time (approximately3 minutes) of the specimen where the concentration of nitrogen issubstantially 0%, in other words, the specimen in which the Ta layer isformed, was longer than the reaction time (approximately 2 minutes) ofthe specimen in which the Pt layer is formed. However, according to theincrease in the concentration of nitrogen, the reaction time of theTa_(1−x)N_(x) layer became shorter, and the reaction time of theTa_(1−x)N_(x) layer became shorter than that of the comparison specimen.Then, the reaction time is shortest in the vicinity of 40% ofconcentration of nitrogen.

The reaction time of the Ta_(1−x)N_(x) layer rapidly increased accordingto the increase in the concentration of nitrogen. The reaction timebecame substantially equal to that of the comparison specimen when theconcentration of nitrogen became approximately 48%.

Then, the reaction time of the Ta_(1−x)N_(x) layer became substantiallytwice as long as that of the comparison specimen when the concentrationof nitrogen was approximately 50%. The reaction time continued toincrease thereafter. Here, it may be difficult to create a specimenwhose concentration of nitrogen is higher than approximately 52% by thesputtering method.

The results disclosed above reveal that forming an air-bridge wiringwith the Ti/TaN/Au laminated film supported by TaN whose concentrationof nitrogen is not less than approximately 48% allows formation of anair-bridge wiring with higher electromigration resistance.

For this reason, the inventor experimentally created a multi-gatetransistor provided with air-bridge wirings formed of the Ti/TaN/Aulaminated film. Hereinafter, a manufacturing process of the air-bridgewirings will be disclosed.

FIG. 4 illustrates a cross sectional view of an air-bridge wiringstructure of the multi-gate transistor that was used in the experiments.In the multi-gate transistor, a source electrode crosses over a gateelectrode with an overhead crossing.

A common electrode 14″ that forms a gate electrode is formed over anAlGaN inactive region 36 caused to have a high resistance by ionimplantation into an n-type AlGaN carrier supply layer 28. In addition,an air-bridge wiring 12 (the source electrode) crosses over the commonelectrode 14″ (the gate electrode), which is covered with a protectionfilm 32, with a space 38 being interposed between the common electrode14″ and the air-bridge wiring 12.

Here, the air-bridge wiring 12 includes a Ti layer 40, a TaN layer 42, afirst Au layer 44, a second Au layer 46, and a third Au layer 48.

Next, the manufacturing process of the air-bridge wiring structure inthe experimentally created multi-gate transistor will be disclosed.

First, an unintentionally doped-GaN (UID-GaN) channel layer 26, then-type AlGaN carrier supply layer 28, a UID-GaN cap layer (not shown)are laminated in turn over an SiC substrate 24, and a substrate 50 withan ohmic electrode 34 being formed thereover is prepared (see FIG. 5A).

Here, the ohmic electrode 34 is formed over a semiconductor regionintended for the source and the drain. Moreover, the UID-GaN cap layeris removed except for a semiconductor region for a channel (thereforenot shown in FIG. 5A). In addition, the n-type AlGaN carrier supplylayer 28 exposed over an area for forming the common electrode 14″ iscaused to have a high resistance by the ion implantation, and the n-typeAlGaN carrier supply layer 28 becomes the AlGaN inactive region 36.

Next, a photo-resist film 52 for lift-off is formed over the substrate50. Thereafter, a Ni/Au laminated film 54 for the gate electrode isdeposited (see FIG. 5B). Then, the photo-resist film 52 for lift-off isremoved, and a gate electrode 22 is formed. The Ni/Au laminated film 54forms a Schottky barrier on the UID-GaN cap layer.

Then, an SiN film 56 having a thickness of approximately 500 nm andintended for a protection film is deposited over the substrate 50 (seeFIG. 5C).

Then, a photo-resist film 52′ is formed over an area for forming theprotection film 32 (see FIG. 5D). The SiN film 56 is removed by dryetching in which the photo-resist film 52′ is used as an etching maskand the protection film 32 is formed (see FIG. 5E).

Then, a photo-resist film (a mask) 58 for air-bridge formation is formedover the protection film 32 at a location where an intersection of thegate electrode and the source electrode is to be located (see FIG. 5F).

Then, the Ti layer 40 having a thickness of, for example, approximately10 nm, the TaN layer 42 having a thickness of, for example,approximately 200 nm, and the first Au layer 44 having a thickness of,for example, approximately 50 nm are deposited in turn over thesubstrate 50 by, for example, the sputtering method (see FIG. 5G). Forexample, the Ti layer 40 and the first Au layer 44 may be deposited bythe sputtering method in which Ti and Au are used as targets. A sputtergas may be, for example, Ar. In addition, for example, TaN may bedeposited by a reactive sputtering method in which Ta is used as atarget. Here, a reactive gas may be, for example, N₂.

Then, the second and the third Au layers 46 and 48 are formed thickly byelectrolytic plating with the first Au layer 44 as a seed film and aphoto-resist film (not shown), which has openings for forming a sourceelectrode and a drain electrode, as a plating mask. The combinedthickness of the second and the third Au layers 46 and 48 may be, forexample, 1 μm to 2 μm.

Thereafter, the plating mask is removed, and a sputter film beingexposed in between respective electrodes is removed by milling.

The photo-resist mask 58 for air-bridge formation is removed, forexample, with a microwave asher in which the reaction gas may be, forexample, oxygen, and a multi-gate transistor 2 is formed (see FIG. 5H).

As result of experimentally creating the multi-gate transistor disclosedabove, there is a possibility that the space 38, which separates aTi/TaN/Au sputter film 68 and the gate electrode 22, is not formed dueto the melting of the photo-resist film 58 for air-bridge formation inthe process of the Ti/TaN/Au sputter film 68 deposition.

Consequently, the inventor measured a rise in substrate temperatureduring the deposition of the TaN layer. As a result of measurement, itwas revealed that the substrate temperature may exceed approximately150° C., which is the melting temperature of the photo-resist.

Such rise in temperature results from the heat of reaction of Tasputtered from a Ta target and nitrogen gas.

It is expected that use of the Ti/TaN/Au laminated film allows formationof the air-bridge wirings with high electromigration resistance.However, due to the rise in substrate temperature during the TaNdeposition, it may be difficult to form the air-bridge wirings.

First Embodiment

Reducing the thickness of the TaN layer to be formed may suppress therise in the substrate temperature associated with the deposition of aTaN layer. This is because the time in which the substrate 50 is exposedto the heat of reaction of TaN is shortened. However, a metal layerhaving a certain amount of film thickness (for example, 100 nm to 200nm) is desirable to stably support an Au layer (the first to third Aulayers 44, 46, and 48). Therefore, reducing the TaN layer results in thedegradation of the strength of air-bridge wirings.

As a result of various studies made by the inventor, it was revealedthat formation of a Ta/TaN laminated film where the TaN layer waslaminated over a Ta layer reduced the rise in substrate temperature incomparison to a case where a TaN film having a substantially equal filmthickness is formed.

In the above case, since the time in which the substrate is exposed tothe heat of reaction of TaN is shortened, the rise in the substratetemperature may be reduced. Furthermore, the inventor also revealed thatelectromigration resistance of the Ta/TaN laminated film was higher thanthat of the TaN film, as disclosed hereinafter. In consequence, theinventor manufactured a multi-gate transistor by using the air-bridgewirings formed of a Ti/Ta/TaN/Au laminated film.

(1) Structure

FIG. 6 illustrates a perspective view of a multi-gate transistoraccording to a first embodiment. FIG. 7 illustrates a cross sectionalview that is viewed along the line A-A in FIG. 6 from the directionshown by the arrows.

FIG. 8 illustrates a cross sectional view that is viewed along the lineB-B in FIG. 6 from the direction shown by the arrows.

A multi-gate transistor 2 according to the first embodiment has astructure in which four (4) GaN-HEMTs 4 are arranged in a line. Each ofthe GaN-HEMTs 4 adjacent to one another shares one semiconductor regionas a source 6 or a drain 8 (see FIG. 7). One electrode 10 and anotherelectrode 10′ are formed over each semiconductor region.

Moreover, two electrodes 10 formed over the sources 6 are coupled to onecommon electrode 14 through two air-bridge wirings 12, and a comb-shapedsource electrode 16 are formed (shown in FIG. 6).

Also, three electrodes 10′ formed over the drains 8 are coupled to theone common electrode 14′ illustrated in FIG. 6, and a comb-shaped drainelectrode 18 is formed. Moreover, four electrodes 10″ that are eachformed over a channel and serve as gates 20 are coupled to one commonelectrode 14″, and a comb-shaped gate electrode 22 illustrated in FIG. 6is formed.

As disclosed above, if the three types of comb-shaped electrodes areformed in one location, it may be inevitable that two electrodes, forexample, the gate electrode 22 and the source electrode 16 cross eachother (see FIG. 6).

For this reason, to avoid contact between electrodes, an air-bridgewiring structure is formed at an intersecting location where oneelectrode crosses over the other electrode, with a space beinginterposed between the electrodes, with an overhead crossing.

The common electrodes 14, 14′, and 14″ may be used as coupling pads toexternal circuits. Consequently, the source electrode 16, the drainelectrode 18, and a gate electrode 22 function as so-called“electrodes.” On the other hand, since the source electrode 16, thedrain electrode 18, and the gate electrode 22 are provided to securepaths for electrical signals input and output to and from respectiveGaN-HEMT terminals (sources, drains, and gates), the source electrode16, the drain electrode 18, and the gate electrode 22 also function as“wirings.” That is to say, although the source electrode 16, the drainelectrode 18, and the gate electrode 22 are called “electrodes”, thesource electrode 16, the drain electrode 18, and the gate electrode 22may also be used as wirings.

Here, a structure of the multi-gate transistor 2 will be disclosed.

As illustrated in FIG. 7, the multi-gate transistor 2 according to thefirst embodiment has a structure in which an unintentionally doped-GaN(UID-GaN) channel layer 26 is laminated over a SiC substrate 24.Moreover, an n-type AlGaN carrier supply layer 28 and a UID-GaN caplayer 30 are laminated over the UID-GaN channel layer 26 in turn,according to the first embodiment.

In addition, Ti/Al ohmic electrodes 34 where aluminum (Al) is laminatedover Ti are formed over the n-type AlGaN carrier supply layer 28.Moreover, either the electrodes 10 that form the source electrodes 16 orthe electrodes 10′ that form the drain electrodes 18 are formed over theohmic electrodes 34. Furthermore, the electrodes 10″ that form the gateelectrodes 22 are formed over the UID-GaN cap layers 30 laminated overthe UID-GaN channel layer 26 which serve as the channels.

Moreover, the gate electrode 22 in FIG. 6 (the electrodes 10″) iscovered with a protection film 32 made of, for example, silicon nitride(SiN). The protection film 32 protects the gate electrode 22 fromcontamination by dust and reduces if not prevents degradation of outputsignals (drain current waveforms).

FIG. 8 illustrates a cross sectional view that is viewed along the lineB-B in FIG. 6 from the direction shown by the arrows. That is to say,FIG. 8 illustrates a cross sectional view of the air-bridge wiringstructure where the source electrode 16 crosses over the gate electrode22 with an overhead crossing.

The common electrode 14″ that forms the gate electrode 22 is formed overan AlGaN inactive region where ion implantation causes the n-type AlGaNcarrier supply layer 28 to have a high resistance. In addition, anair-bridge wiring 12 (that is to say, the source electrode 16) crossesover the common electrode 14″ (that is to say, the gate electrode 22)covered with the protection film 32, with a space being interposedbetween the air-bridge wiring 12 and the common electrode 14″.

Here, the air-bridge wiring 12 is formed by laminating a Ti layer 40, aTa layer 70, a TaN layer 72, a first Au layer 44, a second Au layer 46,and a third Au layer 48A concentration of nitrogen in TaN is, forexample, approximately 50%. The thicknesses of the Ta layer and the TaNlayer are, for example, approximately 100 nm. Note that the commonelectrodes 14 and 14′ that form the source electrodes 16 and the drainelectrodes 18 are also formed over the AlGaN inactive region.

(2) Manufacturing Process

FIGS. 9A to 9I are cross sectional views that illustrate a process offorming an air-bridge wiring 12 according to the first embodiment.

The UID-GaN channel layer 26, the n-type AlGaN carrier supply layer 28,and the UID-GaN cap layer (not shown) are laminated in turn over the SiCsubstrate 24, and the substrate 50 with the ohmic electrode 34 beingformed thereover is prepared (see FIG. 9A). Here, the ohmic electrode 34is formed over the semiconductor region that is intended to serve as thesources and the drains. Moreover, the UID-GaN cap layer is removedexcept for the semiconductor regions that are intended to serve aschannels. In addition, the n-type AlGaN carrier supply layer 28 exposedon an area intended for forming the common electrode 14″ becomes anAlGaN inactive region 36 having a high resistance by ion implantation.

A photo-resist film 52 for lift-off is formed over the substrate 50.Then, a Ni/Au laminated film 54 that is intended to serve as the gateelectrodes 22 is deposited (see FIG. 9B). The photo-resist film 52 forlift-off is removed, and the gate electrode 22 is formed. The Ni/Aulaminated film 54 forms a Schottky barrier on the UID-GaN cap layer (notshown).

An Sin film 56, intended to serve as a protection film 32 and having athickness of, for example, approximately 500 nm, is deposited over thesubstrate 50 (see FIG. 9C). The thickness of the protection film 32 isdesirably, for example, 5 nm to 500 nm.

Thereafter, a photo resist film 52′ is formed over an area intended forforming the protection film 32 (see FIG. 9D). Use of the photo-resistfilm 52′ as an etching mask allows formation of the protection film 32by removing the SiN film 56 with dry etching (see FIG. 9E).

A photo-resist film (a mask) 58 for air-bridge formation is formed overthe protection film 32 at the location where the intersection of thegate electrode and the source electrode is to be located (see FIG. 9F).

The Ti layer 40, the Ta layer 70, the TaN layer 72 (with, for example, aconcentration of nitrogen of approximately 50%), and the first Au layer44 are deposited in turn over the substrate 50 (see FIG. 9G). Here, thethicknesses of the Ti layer 40, the Ta layer 70, the TaN layer 72, andthe first Au layer 44 are, for example, approximately 10 nm,approximately 100 nm, approximately 100 nm, and approximately 50 nm,respectively. Note that the Ti layer 40 is provided for promotingadhesiveness of the Ta/TaN laminated film and may be substituted byother metal layers, for example, a Cr layer.

The Ti layer 40, the Ta layer 70, and the first Au layer 44 are, forexample, deposited with a sputtering method in which Ti, Ta, and Au, areused as targets, respectively. A sputter gas is, for example, Ar. On theother hand, the TaN layer 72 is, for example, deposited with a reactivesputtering method in which Ta is used as a target. A reactive gas is,for example, N₂. At this point of time, since the deposited TaN layer isthin enough and has a thickness of, for example, approximately 100 nm,the temperature of the substrate 50 may not increase above approximately150° C. Therefore, the photo-resist film 58 for air-bridge formation maynot melt.

Note that it may be possible to provide the photo-resist film 58 forair-bridge formation with a UV curing to make the cross section thereofmore round before forming the Ti layer 40.

As the cross section of the photo-resist film 58 for air-bridgeformation becomes more rectangular in shape, the coverage of aTi/Ta/TaN/Au sputter film 76 becomes smaller. Note that the “coverage”means the ratio of the thickness of a metal film deposited on the topsurface of the resist to the thickness of the metal film deposited onside faces of the resist. Consequently, making the cross section of thephoto resist film 58 for air-bridge formation more rounded in shapeallows for easier formation of the air-bridge.

Using the first Au layer 44 as a seed film, the second and the third Aulayers 46 and 48 are formed by electroplating using, as a plating mask,a photo-resist film (not shown) with openings for the areas for thesource and drain electrodes.

Thicknesses of the second and the third Au layers 46 and 48 are, forexample, 1 μm to 2 μm in total.

Thereafter, the plating mask is removed, and the sputter film beingexposed between respective electrodes is removed by milling (FIG. 9H).

The photo-resist film 58 for the air-bridge formation is removed with amicrowave asher to form the space 38, in which the reactive gas is, forexample, oxygen (see FIG. 9I).

Here, since the Ta/TaN laminated layer is thick enough and has athickness of approximately 200 nm, for example, the air-bridge wiringmay not collapse even with the removal of the photo-resist film 58 forthe air-bridge formation.

The method of manufacturing the multi-gate transistor 2 includes theprocess disclosed above.

Hereinafter, the method of manufacturing the multi-gate transistor 2according to the first embodiment includes the following process.

According to the manufacturing method, the photo-resist film 58 for theair-bridge formation is formed over the substrate 50 where asemiconductor element (GaN-HEMT 4) is formed, and the photo-resist film58 for the air bridge formation is coupled to the semiconductor elementand linearly extends such that the top surface of a first wiring (thegate electrode 22) crossing the other wiring is covered at the locationwhere the intersection is intended.

In addition, according to the manufacturing method, the titanium layer40, the tantalum layer 70, the tantalum nitride layer 72, and the firstgold layer 44 are laminated in turn, and a second wiring (the sourceelectrode 16), electrically coupled to the semiconductor element andextending from one end of the photo-resist film 58 to the other endthereof such that a the second wiring crosses over the photo-resist film58, is formed as the other wiring.

Moreover, according to the manufacturing method, the photo-resist film58 is removed after forming the second wiring.

Here, the formation of the tantalum nitride layer 72 is completed beforethe photo-resist film 58 is melted.

(3) Electromigration Resistance

Next, the evaluation results of the electromigration resistance for theair-bridge wirings according to the first embodiment are disclosed.

The electromigration resistance was evaluated by a method similar to theaccelerated thermal degradation test disclosed above.

Evaluation specimens used for the evaluation were created by laminatinga Ti layer having a thickness of approximately 10 nm, a Ta layer havinga thickness of approximately 100 nm, a TaN layer having a thickness ofapproximately 100 nm and a concentration of nitrogen of approximately50%, and an Au layer having a thickness of approximately 50 nm to 300 nmin turn over the Ti/Al laminated film. The method of forming films withthe Ti layer, the Ta layer, the TaN layer, and the Au layer is similarto, if not the same as, the method of forming the films disclosed above.On the other hand, the heating temperature is approximately 450° C.

FIG. 10 is a graph representing results of the accelerated thermaldegradation tests of the first embodiment. For comparison, the graph ofFIG. 10 represents the results of Ti/TaN/Au and Ti/Pt/Au (see FIG. 3)and the results of the first embodiment. The bold dotted line “Ta/Tan”represents the reaction time (approximately 6 minutes) of the aboveevaluation specimen where a Ti/Ta/TaN laminated film is formed betweenthe Ti/Al laminated film and the Au layer (the thicknesses of the Talayer and the TaN layer are approximately 100 nm, respectively.).

On the other hand, the curved line “TaN” represents the reaction time ofthe specimen where a Ti/Ta_(x)N_(−x) laminated film (0≦X<1) is formedbetween the Ti/Al laminated film and the Au layer (note that thethickness of the Ta_(x)N_(1−x) layer is approximately 200 nm.).

In addition, the dotted line “TaN (t=200 nm)” represents the reactiontime (approximately 4 minutes) of the specimen formed of the Ti/TaNlaminated film in which the TaN layer having a thickness ofapproximately 200 nm and a concentration of nitrogen of approximately50% is laminated over the Ti layer having a thickness of approximately10 nm between the Ti/Al laminated film and the Au layer.

On the other hand, the dotted line “Pt” represents the reaction time(approximately 2 minutes) of the specimen where the Ti/Pt laminated filmis formed between the Al layer and the Au layer (the thickness of the Ptlayer is approximately 200 nm.).

Here, since the Ti layer is included each of the Ti/Ta/TaN laminatedfilm, Ti/TaN laminated film, and Ti/Pt laminated film, the Ti layer doesnot cause differences in the reaction times. Hence, the metal layersthat affect the differences in electromigration resistances ofrespective laminated films are the Ta/TaN laminated layer, TaN layer,and the Ti/Pt layer.

As is apparent from the graph of FIG. 10, the reaction time of thespecimen where the Ti/Ta/TaN laminated film is formed is approximately 3times longer than that of the specimen where Ti/Pt laminated film isformed, and approximately 1.5 times longer than that of the specimenwhere Ti/TaN (the thickness of approximately 200 nm) is formed.

These facts indicate that the electromigration resistance of the Ta/TaNlaminated film surpasses that of the TaN layer having a thickness ofapproximately 200 nm, and the electromigration resistance of the Ta/TaNlaminated film is much higher than that of the conventional Pt layer.

Note that the graph of FIG. 10 also represents the result of theaccelerated thermal degradation test implemented on the Ti/TaN laminatedfilm as the dotted line “TaN (t=100 nm)” where the TaN layer having athickness of approximately 100 nm is laminated over the Ti layer. Asrepresented in the graph of FIG. 10, the reaction time of the specimenwhere TaN having a thickness of approximately 100 nm is formed thereinis approximately 1 minute, and this value is only a half of that of thespecimen where the Pt layer having a thickness of approximately 200 nmis formed.

As disclosed above, the electromigration resistance of the Ta/TaNlaminated film surpasses the electromigration resistance of the TaNlayer having a similar, if not the same, film thickness. It may beassumed that, as a reason thereof, growth of the TaN layer over the Talayer causes a layer structure (composition) of the TaN layer to change,and the density of the TaN layer is increased. For example, it may beassumed that the growth of the TaN layer over the Ta layer results inmixed growth of large and small TaN grains, so that the small TaN grainsfill gaps among the large TaN grains, and as a result, the density ofthe TaN layer is increased.

(4) Operation

Hereinafter, an operation of the multi-gate transistor 2 according tothe first embodiment will be disclosed.

To operate the multi-gate transistor 2, each of the electrodes iscoupled in the following manner. The source electrode 16 of themulti-gate transistor 2 is electrically coupled to a ground. The drainelectrode 18 is electrically coupled to a power supply via a loadresistor. A signal is input to the gate electrode 22 in the abovecondition.

A signal voltage input to the gate electrode 22 is supplied to the gate20 of each GaN-HEMT 4, and the input signal causes a drain current ofthe each GaN-HEMT 4 to vary based on a mutual conductance. The draincurrents are supplied to the load resistor (the GaN-HEMT 4 according tothe first embodiment is for example an n-channel field effecttransistor) after being merged by the common electrode 14′ (forming thedrain electrode 18). (The GaN-HEMT 4 of this embodiment is an n-channelfield-effect transistor.

Consequently, the current is supplied from the load resistor to thedrain. In other words, the current is not a “drain current,” butelectron flows flowing from each of the drains are merged and suppliedto the load resistor.)

Here, since the drain currents are merged by the common electrode 14′,the amount of current flowing through the load resistor is made largerin proportion to the number of GaN-HEMTs 4 forming the multi-gatetransistor 2.

At this point of time, the source currents of the respective GaN-HEMTs 4are discharged to the ground after passing through the air-bridge wiring12 and being merged by the common electrode 14. As a result, the currentflowing through the air-bridge wiring 12 varies depending on the signalvoltages applied to the gate. An average value of the currents flowingthrough the air-bridge wiring 12 exceeds, for example, approximately1×10⁵ A/cm₂.

However, the air-bridge wiring 12 is supported with a Ta/Tan laminatedfilm 74 having a high electromigration resistance in the multi-gatetransistor 2 according to the first embodiment. By virtue thereof, thepossibility of disconnection of the air-bridge wiring 12 may beeffectively reduced.

Meanwhile, a large current is caused to flow through each of the sources6 of respective GaN-HEMTs 4. At that time, there is a possibility thatelectromigration may occur at the Au layer that forms the electrode 10′of the source electrode 16 and may cause a reaction with the Ti/Al ohmicelectrode 34 provided over the source 6. However, as is apparent fromthe manufacturing process disclosed above, the Ta/TaN laminated film 74having a high electromigration resistance is provided to separate the Aulayers (the first to the third Au layers 44, 46, and 48) forming theelectrode 10′ and the Ti/Al ohmic electrode 34. Consequently, thepossibility of reaction between both layers may be effectively reduced.

Furthermore, the resistance value of the Ta/TaN laminated film 42 isonly approximately 60% of the resistance value of the TaN layer having asimilar, if not the same, thickness.

Meanwhile, the signal voltage input to the gate electrode 22 propagatesthrough the gate electrode 22 (the first wiring). The gate electrode 22crosses the air-bridge wiring 12. However, since the space 38 separatesboth the gate electrode 22 and the air-bridge wiring 12, cross talkbetween both due to a parasitic capacitance is small. This fact isshared with the following embodiments disclosed hereinafter.

Since the parasitic capacitance is small, the multi-gate transistor 2according to the first embodiment is capable of being operated at, forexample, several GHz.

Second Embodiment

A second embodiment relates to a semiconductor device and amanufacturing method thereof where the Ta/TaN layer forming theair-bridge wiring 12 in the semiconductor device according to the firstembodiment is substituted by a Ta/Ti/TaN laminated film with aconcentration of nitrogen that is, for example, approximately 50%.

(1) Structure

A structure of the semiconductor (having the air-bridge wiring 12)according to the second embodiment is similar to, if not the same as,the semiconductor device disclosed in the first embodiment, except for astructure of the source electrode 16 and the drain electrode 18.

The source electrode 16 and the drain electrode 18 are formedconcurrently as disclosed in the first embodiment. In consequence, thesource electrode 16 and the drain electrode 18 include similar, if notidentical, laminated structures. Therefore, the structure of thesemiconductor device (the multi-gate transistor) according to the secondembodiment will be disclosed hereinafter by especially referring to theair-bridge wiring 12 (that forms a part of the source electrode 16).

FIG. 11 illustrates a cross sectional view of the air-bridge wiring 12that forms the multi-gate transistor according to the second embodiment.

The Ta/TaN layer 74 forming the air-bridge wiring 12 of the firstembodiment is substituted with a Ta/Ti/TaN laminated film 80 in theair-bridge wiring 12 according to the second embodiment (see FIG. 11).Here, the Ta/Ti/TaN laminated film 80 is formed by lamination of the Talayer 70, a Ti layer 78, and the TaN layer 72 in turn.

Moreover, the thicknesses of the Ta layer 70, the Ti layer 78, and theTaN layer 72 are, for example, approximately 100 nm, approximately 10nm, and approximately 100 nm, respectively. The thicknesses of the othermetal layers may be similar to, if not the same as, that of thecorresponding metal layers of the multi-gate transistor 2 according tothe first embodiment. Note that the concentration of nitrogen in the TaNlayer is, for example, approximately 50%.

That is to say, in the multi-gate transistor according to the secondembodiment, another Ti (titanium) layer 78 is provided between the Ta(tantalum) layer 70 and the TaN (tantalum nitride) layer 72 that formthe air-bridge wiring 12 in the multi-gate transistor 2 according to thefirst embodiment.

(2) Manufacturing Process

The manufacturing process of the multi-gate transistor according to thesecond embodiment is similar to that of the multi-gate transistor 2according to the first embodiment. Hereinafter, the manufacturingprocess that is different from that of the first embodiment will bedisclosed.

The manufacturing process of the second embodiment is similar to, if notthe same as, that of the first embodiment except that a Ti/Ta/Ti/TaN/Ausputter film 82 is formed instead of the Ti/Ta/TaN/Au sputter film 74.For example, the Ti layer 78 provided between the Ta layer 70 and theTaN layer 72 is formed with the sputtering method in which Ti is used asa target. The sputter gas may be, for example, Ar.

That is to say, in the semiconductor manufacturing method according tothe second embodiment, the TaN (Tantalum nitride) layer is laminatedafter the other Ti (Titanium) layer has been laminated over the Ta(Tantalum) layer, based on the manufacturing method disclosed in thefirst embodiment.

(3) Electromigration Resistance

Next, the evaluation results of the electromigration resistance for theTa/Ti/TaN laminated film 80 based on the accelerated thermal degradationtest will be disclosed. Specimens used for the evaluations are specimenswhere the Ti/Ta/Ti/TaN/Au laminated film is laminated over the Ti/Allaminated film. Here, the thickness of the Au layer is, for example, 50nm to 300 nm.

FIG. 12 is a graph representing the results of the accelerated thermaldegradation tests implemented on the Ta/Ti/TaN laminated film 80according to the second embodiment. For comparison, the graph of FIG. 12also represents the results of accelerated thermal degradation testsdisclosed in FIGS. 3 and 10.

The bold dotted line “Ta/Ti/TaN” represents the reaction time(approximately 8 minutes) of the specimen where the Ta/Ti/TaN laminatedfilm 80 according to the second embodiment is formed. As is apparentfrom the graph of FIG. 12, the reaction time of the specimen where theTa/Ti/TaN laminated film 80 is formed is approximately four times longerthan the specimen where the Pt layer is formed and approximately 1.3times longer than the specimen where the Ta/TaN laminated film 74according to the first embodiment is formed.

These facts reveal that the electromigration resistance of the Ta/Ti/TaNlaminated film 80 surpasses that of the Ta/TaN laminated film 74according to the first embodiment, and the electromigration resistanceof the Ta/Ti/TaN laminated film 80 is high enough compared to that ofthe conventional Pt layer.

The reason why the electromigration resistance of the Ta/Ti/TaNlaminated film 80 is higher than that of the Ta/TaN laminated film 74according to the first embodiment is thought to be because the densityof the TaN layer is increased by providing the Ti layer between the Talayer and the TaN layer.

Note that the operation of the multi-gate transistor according to thesecond embodiment is similar to, if not the same as, the multi-gatetransistor according to the first embodiment except that theelectromigration resistance of the air-bridge wiring 12 becomes higher.

Third Embodiment

A third embodiment relates to a semiconductor device where theprotection film covering the source electrode is removed under theair-bridge wiring in the semiconductor device according to the firstembodiment and relates to a manufacturing method thereof.

(1) Structure

FIG. 13 illustrates a perspective view of a multi-gate transistor 84according to the third embodiment. FIG. 14 illustrates a cross sectionalview that is viewed along the line B-B in FIG. 13 from the directionshown by the arrows.

As illustrated in FIG. 13, the multi-gate transistor 84 according to thethird embodiment is different from the multi-gate transistor 2 accordingto the first embodiment in that the common electrode 14″ that isincluded the gate electrode 22 is not covered with a protection film 32,that is to say, the common electrode 14″ is exposed. Note that thecommon electrode 14″ of the gate electrode 22 is provided under theair-bridge wiring 12.

The structure thereof is similar to, if not the same as, the multi-gatetransistor 2 according to the first embodiment except for the commonelectrode 14″. Thus, the description on the structure will be reduced oromitted except for that of the common electrode 14″.

As illustrated in FIG. 6, the common electrode 14″ is covered with theprotection film 32 even under the air-bridge wiring 12, in themulti-gate transistor 2 according to the first embodiment. Consequently,there is a portion where a dielectric film, such as SiN or the like, isprovided between the air-bridge wiring 12 and the common electrode 14″.Due to the dielectric film, the parasitic capacitance between the sourceelectrode 16, which includes the air-bridge wiring 12, and the gateelectrode 22, which includes the common electrode 14″, is made larger.

As illustrated in FIG. 13, the protection film 32, which is providedunder the air bridge wiring 12 and covering the common electrode 14″, isremoved in the multi-gate transistor 84 according to the thirdembodiment, and a reduction in the parasitic capacitance may beachieved. That is to say, as illustrated in FIG. 14, the protection film(SiN film) is removed at the portion where a first wiring (the sourceelectrode 16 that includes the air-bridge wiring 12) crosses over asecond wiring (a gate electrode 22 that includes the common electrode14″).

For the above reason, signal interference between the gate electrode 22and the source electrode 16 may become smaller in the multi-gatetransistor 84 according to the third embodiment. Therefore, themulti-gate transistor 84 according to the third embodiment is capable ofoperating faster than the multi-gate transistor 2 according to the firstembodiment.

The protection film 32 serves to protect the gate electrode 22 fromcontamination by dust and reduce, if not prevent, degradation of theoutput signals (drain current waveforms).

However, a Ni/Au electrode 10″ that is included in the gate 20 may becovered with the protection film 32 so as to reduce, if not prevent, thedegradation of the output signals. Hence, the degradation of the outputsignals may not occur even if the protection film 32 covering the commonelectrode 14″ is removed as disclosed in the third embodiment.

Note that the Ni/Au common electrode 14″ is covered with the protectionfilm 32 and this serves to protect a Ni layer forming the commonelectrode 14″ from oxidation in the multi-gate transistor 2 according tothe first embodiment.

In addition, note that the Ta/TaN layer that forms the air-bridge wiring12 may be substituted with Ta/Ti/TaN laminated film, as disclosed in thesecond embodiment.

(2) Manufacturing Process

FIGS. 15A to 15D illustrate cross sectional views that indicate amanufacturing process of forming the protection film 32 to theair-bridge wiring 12 according to the third embodiment.

Here, FIGS. 15A to 15D are views along the line B-B in FIG. 13 from thedirection shown by the arrows that illustrate the progress of themanufacturing process.

As disclosed hereinafter, the manufacturing process according to thethird embodiment is similar to, if not the same as, the manufacturingprocess according to the first embodiment except for formation of theprotection film and the photo-resist film (mask) for air-bridgeformation.

The substrate 50 is prepared where the plural GaN-HEMTs 4 that isincluded in the multi-gate transistor 84, which include ohmicelectrodes, are formed.

Next, the gate electrode 22, for example Ni/Au gate electrode 22, isformed.

These processes are similar to, if not the same as, the correspondingprocesses in the first embodiment and detailed description will bereduced or omitted.

First, a SiN film having a thickness of, for example, approximately 500nm and prepared for serving as the protection film 32 is deposited overthe SiC substrate 24 where the gate electrode 22 has been formed. Here,the preferable thickness of the SiN film is, for example, approximately5 nm to 500 nm.

Thereafter, the photo-resist film is formed over an area intended forforming the protection film. Here, the third embodiment is differentfrom the first embodiment in that the common electrode 14″ is notcovered with the photo-resist film.

The SiN film is removed by dry etching where the photo-resist filmserves as an etching mask, and the protection film 32 is formed (seeFIG. 13).

In consequence, the protection film (SiN film) is removed at the areaintended for forming an intersection where the source electrode 16 (thesecond wiring) crosses the gate electrode 22 (the first wiring). Inother words, the protection film (SiN film) is removed under the areawhere formation of the air-bridge wiring 12 is to be located. (see FIG.15A).

Next, the photo-resist film 58 for air-bridge formation, which extendslinearly such that the top surface and the side faces of the sharedelectrode 14″ are covered, is formed over the substrate 24.

Then, as illustrated in FIG. 15C, the Ti/Ta/TaN/Au sputter film 76(here, concentration of nitrogen in TaN is, for example, approximately50%.) is formed over the substrate 24 where the photo-resist film 58 forair-bridge formation has been formed.

Then, the second and the third Au layers 46 and 48 are formed over theTi/Ta/TaN/Au sputter film 76 for example by a plating method.

Then, as illustrated in FIG. 15D, the photo-resist film 58 forair-bridge formation is removed, and the multi-gate transistor 84 isformed.

(3) Operation

Operations of the multi-gate transistor 84 according to the thirdembodiment are substantially similar to, if not the same as, those ofthe multi-gate transistor 2 according to the first embodiment. Note,however, that the multi-gate transistor 84 according to the thirdembodiment operates with higher frequencies in comparison to themulti-gate transistor 2 according to the first embodiment because theparasitic capacitance between the source electrode 16 and the gateelectrode 22 is smaller than that of the multi-gate transistor 2.

Fourth Embodiment

A fourth embodiment relates to a high frequency amplifier provided withthe multi-gate transistors according to the first to the thirdembodiments.

FIG. 16 illustrates a circuit diagram of the high frequency amplifieraccording to the fourth embodiment.

The high frequency amplifier 86 according to the fourth embodimentincludes any of the multi-gate transistors disclosed in the first to thethird embodiments as a multi-gate transistor 2′.

In addition, the high frequency amplifier 86 according to the fourthembodiment includes a first terminal (an input terminal 88) to which anelectrical signal is input and a second terminal (an output terminal 90)from which an amplified electrical signal is output.

Moreover, the high frequency amplifier 86 according to the fourthembodiment is provided with a first path 92 through which the electricalsignal propagates from the first terminal (the input terminal 88) to thefirst wiring (the gate electrode 22). Furthermore, the high frequencyamplifier 86 according to the fourth embodiment is provided with asecond path 94 through which the electrical signal propagates from athird wiring (the drain electrode 18), electrically coupled to the drain8, to the second terminal (the output terminal 90).

Here, the first path 92 is provided with a first capacitor 97, and thesecond path 94 is provided with a second capacitor 99. In consequence,only a high frequency signal propagates through the first and the secondpaths 92 and 94.

In addition, the high frequency amplifier 86 according to the fourthembodiment includes a third terminal (power supply terminal 96) coupledto a power supply voltage. In addition, the power supply terminal 96 iscoupled to the second wiring (the drain electrode 18) through a firstresistor 98.

Moreover, the power supply terminal 96 is coupled to a coupling pointprovided between the first capacitor 97 and the first wiring (the gateelectrode 22) through a second resistor 100. Furthermore, the couplingpoint is coupled to the groundplane through a third resistor 102. Here,the second and the third resistors 100 and 102 make settings of biaspoints for the multi-gate transistor 2′.

On the other hand, the source electrode 16 is coupled to the groundplane through a circuit where a fourth resistor 104 and a thirdcapacitor 106 are coupled in parallel.

Here, the multi-gate transistor 2′ is included in a given package anddisposed over a printed board along with the first to the fourthresistors and the first to the third capacitors.

The high frequency amplifier 86 as described above operates in thefollowing manner. When an electrical signal is input to the firstterminal 88, the electrical signal propagates through the first path 92and reaches the gate electrode of the multi-gate transistor 2′. Themulti-gate transistor 2′ operates as disclosed in respective embodimentsand amplifies the input electrical signal. The amplified electricalsignal propagates through the second path 94, and the amplifiedelectrical signal is output from the second terminal 90.

Since the high frequency amplifier 86 according to the fourth embodimentis provided with the multi-gate transistor 2′ according to the first tothe third embodiments, disconnection of the air-bridge wiring may beeffectively reduced, if not prevented, even if the high frequencyamplifier 86 is operated with a large current.

Note that the circuit disclosed with reference to FIG. 16 is merely oneexample of the high frequency amplifier that makes use of a multi-gatetransistor according to the first to the third embodiments.

Fifth Embodiment

A fifth embodiment relates to a transmitter provided with the highfrequency amplifier according to the fourth embodiment.

FIG. 17 illustrates a block diagram of a transmitter 108 according tothe fifth embodiment.

A radio transmitter 108 according to the fifth embodiment is providedwith an input terminal 116 to which an electrical signal is input and anoutput terminal 114 to which an antenna 112 is coupled, and the highfrequency amplifier 86 according to the fourth embodiment, asillustrated in FIG. 17. Moreover, the radio transmitter 108 according tothe fifth embodiment is provided with a high frequency signal generationunit 110 (for example, a voltage controlled oscillator [VCO]) thatgenerates and outputs a modulated high frequency signal based on theinput signal.

Moreover, the input terminal 116 is coupled to an input terminal of thehigh frequency signal generation unit 110 in the radio transmitter 108according to the fifth embodiment. In addition, an output terminal ofthe high frequency signal generation unit 110 is coupled to a firstterminal (the input terminal 88) of the high frequency amplifier 86 andthe output terminal 114 is coupled to a second terminal (the outputterminal 90) of the high frequency amplifier 86.

The transmitter 108 operates in the following manner. The electricalsignal input from the input terminal 116 is supplied to the highfrequency signal generation unit 110. The high frequency generation unit110 generates the high frequency signal modulated based on the inputelectrical signal and supplies the high frequency amplifier 86 with themodulated high frequency signal. The high frequency amplifier 86amplifies the supplied high frequency signal and supplies the antenna112 coupled to the output terminal 114 with the amplified high frequencysignal.

Here, since the transmitter 108 is provided with the high frequencyamplifier 86 according to the fourth embodiment, a high-power signal maybe supplied to the antenna. The transmitter according to the fifthembodiment may be, for example, used for base stations of mobiletelephone systems.

[Modifications]

The concentration of nitrogen in the TaN layer is, for example,approximately 50% in the respective embodiments disclosed above.However, the concentration of nitrogen is not limited to the valuedisclosed above. The concentration of nitrogen in the TaN layer maypreferably be more than 48% and less than 52%, or, more preferably, maybe from 49% or higher to 51% or less.

As is apparent from the graph of FIG. 12, if the concentration ofnitrogen is more than 48%, the electromigration resistance of aTa/Ta_(x)N_(1−x) laminated film (or a Ta/Ti/Ta_(x)N_(1−x) laminatedfilm) is higher than the electromigration resistance of conventional Ptlayers. Hence, the concentration of nitrogen may preferably be more than48%. On the other hand, the higher the concentration of nitrogen, themore difficult the formation of the TaN film will be. For example, itmay be difficult to form a TaN film whose concentration of nitrogen isequal to or above 52%. Therefore, the concentration of nitrogen in TaNis preferably equal to or less than 52%.

The protection film is formed with silicon nitride in the respectiveembodiments disclosed above. Note, however, that insulation films,intended for the protection film, may not be limited to the siliconnitride (SiN) film. For example, the protection film may be formed withsilicon dioxide (SiO₂).

The air-bridge wirings are provided over the source electrode in therespective embodiments disclosed above. However, it may be preferablethat the air-bridge wirings are provided over the drain electrode if thedrain electrode crosses the gate electrode. That is to say, the secondwiring may be coupled to the drain instead of the source of the fieldeffect transistor. Moreover, the air-bridge wirings according to themodification may be formed over wirings other than the source electrodeand the drain electrode.

The multi-gate transistors are formed with the GaN-HEMTs in therespective embodiments disclosed above. However, the multi-gatetransistor may be formed with the other field effect transistors such asan InP-HEMT provided with a semi-insulating InP as a substrate, anInGaAs channel layer, or an InAlAs barrier layer into which Si is doped.

Moreover, the air-bridge wirings according to the respective embodimentsdisclosed above may be applicable to semiconductor devices other thanthe multi-gate transistor, for example, field effect transistorsprovided with a plurality of gates.

Furthermore, the semiconductor elements that form the semiconductordevices according to the respective embodiments are field effecttransistors. However, the semiconductor elements may be semiconductorelements other than field effect transistors, for example, bipolartransistors. If a bipolar transistor is used as the semiconductorelement disclosed above, for example, the first wiring is coupled to abase and the second wiring is coupled to an emitter or a collector.

Moreover, although the gate electrode is formed of the Ni/Au laminatedfilm in the respective embodiments disclosed above, the gate electrodemay be formed of other laminated films. In addition, although theinsulation film is etched by the dry-etching so that the protection filmis formed in the respective embodiments disclosed above, the insulationfilm may be etched by other etching methods, such as wet-etching, ionmilling, or the like.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiments of the presentinventions have been described in detail, it should be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

1. A semiconductor device comprising: a first wiring extending in afirst direction; and a second wiring extending in a second directionwhich crosses the first direction and being disposed with a spaceinterposed between the first wiring and the second wiring, and includinga tantalum layer, a tantalum nitride layer formed over the tantalumlayer, and a metal layer formed over the tantalum nitride layer, whereinthe first wiring and the second wiring are not electrically coupled witheach other.
 2. The semiconductor device according to claim 1, whereinthe concentration of nitrogen in the tantalum nitride layer is more than48% and equal to or less than 52%.
 3. The semiconductor device accordingto claim 1 further comprising: a titanium layer formed between thetantalum layer and the tantalum nitride layer.
 4. The semiconductordevice according to claim 1 further comprising: a semiconductor element,wherein the first wiring and the second wiring are coupled to thesemiconductor element and a signal input to the semiconductor elementpropagates through the first wiring.
 5. The semiconductor deviceaccording to claim 4 further comprising: a protection film which coversthe first wiring.
 6. The semiconductor device according to claim 5,wherein the protection film is removed at a portion where the firstwiring crosses the second wiring.
 7. The semiconductor device accordingto claim 5, wherein the protection film is a silicon nitride film or asilicon dioxide film.
 8. The semiconductor device according to claim 4,wherein the semiconductor element is a plurality of transistors, each ofa plurality of sources, gates, and drains of each of the plurality oftransistors extends in the same direction, each of the plurality oftransistors shares at least one of the source and the drain with aplurality of adjacent transistors, the first wiring is electricallycoupled to at least one of the plurality of gates, and the second wiringis electrically coupled to either the source or the drain.
 9. Thesemiconductor device according to claim 8, wherein the transistorincludes gallium nitride as a channel layer.
 10. An amplifiercomprising: a first wiring extending in a first direction; a secondwiring extending in a second direction which crosses the first directionand being disposed with a space interposed between the first wiring andthe second wiring, and including a tantalum layer, a tantalum nitridelayer formed over the tantalum layer, and a metal layer formed over thetantalum nitride layer, wherein the first wiring and the second wiringare not electrically coupled with each other, a plurality of transistorsto which the first wiring and the second wiring are coupled; a thirdwiring; a first terminal which is electrically coupled to the firstwiring and to which an electrical signal is input; a second terminalwhich is electrically coupled to the third wiring and to which theamplified electrical signal is output, wherein each of a plurality ofsources, gates, and drains of each of the plurality of transistorsextends in a same direction, each of the plurality of transistors sharesat least one of the source and the drain with a plurality of adjacenttransistors, the first wiring is electrically coupled to the gate, thesecond wiring is electrically coupled to the source, and the thirdwiring is electrically coupled to the drain.
 11. A radio transmittercomprising: an amplifier; a signal generation unit which generates andoutputs a signal modulated based an input signal; an input terminal towhich an electrical signal is input; an output terminal to which anantenna is coupled, wherein the amplifier comprises: a first wiringextending in a first direction, a second wiring extending in a seconddirection which crosses the first direction and being disposed with aspace interposed between the first wiring and the second wiring, andincluding a tantalum layer, a tantalum nitride layer formed over thetantalum layer, and a metal layer formed over the tantalum nitridelayer, wherein the first wiring and the second wiring are notelectrically coupled with each other, a plurality of transistors towhich the first wiring and the second wiring are coupled, a thirdwiring, a first terminal which is electrically coupled to the firstwiring and to which an electrical signal is input, and a second terminalwhich is electrically coupled to the third wiring and to which theelectrical signal amplified is output; and wherein each of a pluralityof sources, gates, and drains of each of the plurality of transistorsextends in a same direction, each of the plurality of transistors sharesat least one of the source and the drain with the plurality of adjacenttransistors, the first wiring is electrically coupled to at least one ofthe plurality of gates, the second wiring is electrically coupled to thesource, the third wiring is electrically coupled to the drain, the inputterminal is coupled to an input terminal of the signal generation unit,an output terminal of the signal generation unit is coupled to the firstterminal, and the output terminal is coupled to the second terminal. 12.A semiconductor device manufacturing method comprising: forming a firstwiring extending in a first direction over a semiconductor substrate;forming a mask over the first wiring; laminating a tantalum layer, atantalum nitride layer, and a metal layer in turn over the mask andforming a second wiring extending in a second direction which crossesthe first direction; removing the mask after forming the second wiring,wherein a film formation of the tantalum nitride layer is completedbefore the mask is melted.
 13. The semiconductor device manufacturingmethod according to claim 12, wherein the concentration of nitrogen inthe tantalum nitride layer is more than 48% and equal to or less than52%.
 14. The semiconductor device manufacturing method according toclaim 12, wherein a titanium layer is laminated over the tantalum layerand thereafter the tantalum nitride layer is laminated over the titaniumlayer.
 15. The semiconductor device manufacturing method according toclaim 12, wherein the tantalum layer is formed by a sputtering method inwhich tantalum is used as a target, and the tantalum nitride layer isformed by a reactive sputtering method, in which tantalum is used as atarget.
 16. The semiconductor device manufacturing method according toclaim 12, wherein the tantalum layer is formed by a sputtering method inwhich tantalum is used as a target, the tantalum nitride layer is formedby a reactive sputtering method in which tantalum is used as a targetand nitrogen is used as a reactive gas, and the metal layer is formed ofa first gold layer and a second gold layer, the first gold layer beingformed by a sputtering method in which gold is used as a target, and thesecond gold layer being laminated by a plating method.
 17. Thesemiconductor device manufacturing method according to claim 12, whereinthe first wiring and the second wiring are coupled to a semiconductorelement, and the first wiring is the wiring through which a signal inputto the semiconductor element propagates.
 18. The semiconductor devicemanufacturing method according to claim 12 further comprising: coveringthe first wiring with a protection film before the mask is formed. 19.The semiconductor device manufacturing method according to claim 18further comprising: removing the protection film under the second wiringbefore the mask is formed.
 20. The semiconductor device manufacturingmethod according to claim 18, wherein the protection film is either asilicon nitride film or a silicon dioxide film.